FIG. 1 is an exploded perspective view of a conventional wheel unit 9, comprising a circular wheel 90, a rod 92, and a restricting element 91, of a stroller or a push car. The rod 92, in sequence, has a first portion 93, a second portion 94, and a third portion 95, whose diameters are sequentially decreased, and further has a fourth portion 96 whose diameter equals to or slightly smaller than that of the second portion 94. When the rod 92 is inserted into the circular wheel 90 and through a central aperture 97 to the extent that the circular wheel 90 is aligned with the second portion 94, the third portion 95 is gripped by an elastic restricting element 91 and then the circular wheel 90 is secured to the rod 92 to prevent the circular wheel 90 from escaping out of the rod 92.
However, because the spring force of the restricting element 91 is very strong, it is necessary to use tools to assemble and disassemble the circular wheel 90 and the restricting element, which is inconvenient for the consumers. On the other hand, it takes a long time to assemble and disassemble all the elements of the wheel unit sequentially. Furthermore, the restricting element 91 which is small in size is apt to be lost, because the restricting element 91 and the circular wheel 90 are separated from each other after they are dismounted from the wheel unit 9. In addition, the favorable viewing of the wheel unit 9 or even the whole stroller is badly affected due to the protrusion of the restricting element 91 and the fourth portion 96 of the rod 92 beyond the outer surface of the circular wheel 90.